This invention is in the field of fluorinated multilayered carbon nanomaterials, in particular fluorinated multiwalled carbon nanotubes, multilayered carbon nanofibers, multilayered carbon nanoparticles, carbon nanowhiskers and carbon nanorods.
Fluorinated carbons are used commercially as a positive electrode material in primary lithium batteries. Fluorination of graphite allows intercalation of fluorine between the carbon layers. Other industrial applications of fluorinated carbons include use as solid lubricants or as reservoirs for very active molecular oxidizers such as BrF3 and ClF3.
In a lithium/CFx cell, the cell overall discharge reaction, first postulated by Wittingham (1975) Electrochem. Soc. 122:526, can be schematized by equation (1):(CFx)n+xnLinC+nxLiF  (1)
Thus, the theoretical specific discharge capacity Qth, expressed in mAh·g-1, is given by equation (2):
                                          Q            th                    ⁡                      (            x            )                          =                  xF                      3.6            ⁢                          (                              12                +                                  19                  ⁢                                                                          ⁢                  x                                            )                                                          (        2        )            where F is the Faraday constant and 3.6 is a unit conversion constant.
The theoretical capacity of (CFx)n materials with different stoichiometry is therefore as follows: x=0.25, Qth=400 mAh·g-1; x=0.33, Qth=484 mAh·g-1; x=0.50, Qth=623 mAh·g-1; x=0.66, Qth=721 mAh·g-1; and x=1.00, Qth=865 mAh·g-1.
The reactivity of carbon allotropic forms with fluorine gas differs largely owing either to the degree of graphitization or to the type of the carbon material (Hamwi A. et al.; J. Phys. Chem. Solids, 1996, 57 (6-8), 677-688). In general, the higher the graphitization degree, the higher the reaction temperature. Carbon fluorides have been obtained by direct fluorination in the presence of fluorine or mixtures of fluorine and an inert gas. When graphite is used as the starting material, no significant fluorination is observed below 300° C. From 350 to 640° C., two graphite fluorides, mainly differing in crystal structure and composition are formed: poly(dicarbon monofluoride) (C2F)n and poly(carbon monofluoride) (CF)n (Nakajima T.; Watanabe N. Graphite fluorides and Carbon-Fluorine compounds, 1991, CRC Press, Boston; Kita Y.; Watanabe N.; Fujii Y.; J. Am. Chem. Soc., 1979, 101, 3832). In both compounds the carbon atoms take the sp3 hybridization with associated distortion of the carbon hexagons from planar to ‘chair-like’ or ‘boat-like’ configuration. Poly(dicarbon monofluoride) is obtained at ˜350° C. and has a characteristic structure, where two adjacent fluorine layers are separated by two carbon layers bonded by strongly covalent C—C bonding along the c-axis of the hexagonal lattice (stage 2). On the other hand, poly(carbon monofluoride) which is achieved at ˜600° C. has a structure with only one carbon layer between two adjacent fluorine layers (stage 1). Graphite fluorides obtained between 350 and 600° C. have an intermediary composition between (C2F)n and (CF)n and consist of a mixture of these two phases (Kita, 1979). The stage s denotes the number of layers of carbon separating two successive layers of fluorine. Thus a compound of stage 1 has a sequence of stacking of the layers as FCF/FCF . . . , and a compound of stage 2 has the sequence FCCF/FCCF . . . . Both poly(dicarbon monofluoride) and poly(carbon monofluoride) are known to have relatively poor electrical conductivity.
Use of fluorinated carbon nanotubes in batteries has been reported in the patent literature. Japanese Patent publication JP2005285440, Mashushita Electric Ind. Co. Ltd., reports a nonaqueous electrolyte battery including a positive electrode made of a fluorocarbon including fluorinated carbon nanotubes and a negative electrode made of materials which can provide a source of lithium ions.
Reaction of multi-walled carbon nanotubes (MWCNT) with fluorine has been reported in the scientific literature. Hamwi et al. (1997) report fluorination of carbon nanotubes having an outer diameter between 20 and 40 nm prepared by thermal decomposition of acetylene over silica-supported cobalt catalysts. Fluorination at about 500° C. for four hours under pure fluorine atmosphere led to white compounds indicative of complete fluorination. (A. Hamwi, H. Alvergnat, S. Bonnamy, F. Beguin, 1997, Carbon, 35, 723). Touhara et al. (2002) report fluorination of template synthesized carbon nanotubes having an outer diameter of 30 nm at temperatures from 50° C. to 200° C. for 5 days under 1 atm of fluorine gas. (H. Touhara et al., 2002, J. Fluorine Chem, 114, 181-188).
Reaction of carbon fibers with fluorine has also been reported. U.S. Pat. No. 6,841,610 to Yanagisawa et al. reports fluorinated carbon fibers in which the exposed edges of the carbon layers are fluorinated. The pristine carbon fiber starting material had a “herring bone” structure and an average diameter of about 100 nm. The fluorination temperature was reported as 340° C., the fluorine partial pressure as 460 mm Hg, the nitrogen partial pressure as 310 mm Hg and the reaction time as 72 hours. Touhara et al. (1987) reported reaction of elemental fluorine and heat treated vapor-grown carbon fibers having a diameter of approximately 10 microns at temperatures between 330° C. and 614° C. No residual graphite was confirmed for all compounds. The F/C ratios reported ranged between 0.53 (at 345° C.) and 0.99 at 614° C. (Touhara et al., 1987 Electrochemica Acta, Vol. 32, No. 2, 293-298).
Carbon-fluorine intercalation compounds have been also obtained by incorporating other compounds capable of acting as a fluorination catalyst, such as HF or other fluorides, into the gas mixture. These methods can allow fluorination at lower temperatures. These methods have also allowed intercalation compounds other than (C2F)n and (CF)n to be prepared (N. Watanabe et al., “Graphite Fluorides”, Elsevier, Amsterdam, 1988, pp 240-246). These intercalation compounds prepared in the presence of HF or of a metal fluoride have an ionic character when the fluorine content is very low (F/C<0.1), or an iono-covalent character for higher fluorine contents (0.2<F/C<0.5). In any case, the bonding energy measured by Electron Spectroscopy for Chemical Analysis (ESCA) gives a value less than 687 eV for the most important peak of the F1 line and a value less than 285 eV for that of the C1s line (T. Nakajima, Fluorine-carbon and Fluoride-carbon, Chemistry, Physics and Applications, Marcel Dekker 1995 p. 13).
Hamwi et al. have reported room temperature fluorination of MWNT under a gaseous atmosphere of F2, HF and IF5 for about 10 hours. The F/C ratio, determined by mass uptake, was reported as 0.4. Fourier Transform Infrared Spectroscopy spectra were reported to display a broad band centered at about 1100 cm−1, indicating the presence of semi-ionic C—F bonds (Hamwi 1997 ibid.).
U.S. Pat. No. 5,106,606 to Endo et al. report fluorinated graphite fibers having a composition of C5F to C30F. The examples describe room temperature fluorination in the presence of a silver fluoride catalyst.